Disease Surveilance

District Programs & Departments

The District prevents and controls three vectors: mosquitoes, black flies, and midges. It is important to control mosquitoes to reduce their potential as a nuisance and carrier (vector) of diseases. Mosquito-transmitted diseases which are of concern in Southern California are St. Louis encephalitis (SLE), Western Equine encephalomyelitis (WEE), West Nile virus (WNV), malaria, and dog and cat heartworm. Several species of mosquitoes occurring within the District boundaries can transmit these debilitating, and sometimes fatal, diseases to people and animals.

Mosquitoes Known to Occur Within the Greater Los Angeles County Vector Control District

SCIENTIFIC NAME

COMMON NAME

HABITAT

MEDICAL IMPORTANACE

Aedes albopictus

Asian Tiger Mosquito

Lucky bamboo plants in nurseries and man-made containers

Potential vector for dengue fever, WNV, and other encephalitis viruses

Mosquito-Borne Diseases in Los Angeles County

St. Louis encephalitis (SLE) St. Louis encephalitis is a serious viral disease that is spread by infected Culex species mosquitoes. It is one of several mosquito-borne virus diseases that can affect the central nervous system and cause severe complications and death. The virus can be imported into an area by wild birds that may be infected elsewhere. These birds show no symptoms, as they serve as reservoirs of the virus. The infected birds are then fed on by local mosquitoes that can pass the virus on to humans through future bites. The virus cannot be transmitted directly from person-to-person or from birds to people. Most infections are mild and are characterized by headache, fever and nausea. Individuals with weaker immune systems, such as infants and the elderly, are more likely to experience severe illness. Case fatality rates range from 3-30%. There is no specific treatment for SLE. Antibiotics are not effective against viruses, and no effective antiviral drugs have been discovered. Since 1964, 4,478 human cases have been reported in the U.S., with an average of 130 cases per year. Southern California experienced its worst outbreak of human SLE cases from 1984-1986. The combined counties of Los Angeles, Orange, Riverside, and San Diego reported 26 confirmed human cases in 1984. In Los Angeles County, 16 confirmed human cases occurred, seven within the District, with two cases resulting in death. In 1985, three confirmed cases were reported, and only one occurred within the District, followed by three more cases in 1986. SLE is endemic to Los Angeles County.

Western Equine encephalomyelitis (WEE) Western equine encephalomyelitis is a viral disease that is transmitted to horses and humans by infected Culex species mosquitoes. The virus invades the central nervous system, including the spinal cord and brain, causing illness similar to SLE. WEE is known to occur in Los Angeles County.

West Nile virus (WNV) West Nile virus is transmitted to humans, horses, birds, and other animals by the bite of an infected mosquito during blood feeding. A mosquito is infected by biting a bird that carries the virus. The virus is not spread through person-to-person contact but can be passed through blood transfusions. Fewer than one out of 150 people who are bitten by an infected mosquito get severely ill. In most cases people who are infected do not become sick or show symptoms. The virus can, in rare cases, cause encephalitis and death. The mortality rate is 3-15% and varies depending on the age and condition of the health of the victim. The elderly are most at risk for severe cases of the disease. There is no specific treatment for WNV. In a serious case, an individual may be hospitalized to ensure good supportive care. A WNV vaccine is available for horses.

Malaria Malaria, an infectious disease of the red blood cells, is caused by a microscopic protozoan (Plasmodium) entering the red blood cells of a human through the bite of an infected anophele mosquito. Symptoms of malaria include fever and flu-like illness. It may cause anemia and jaundice due to the loss of red blood cells. For most people, symptoms begin 10 days to 4 weeks after infection. Two kinds of malaria, P. vivax and P. ovale can relapse. When these parasites come out of hibernation and begin invading red blood cells, the person will become sick. Malaria can be prevented by taking prophylactic drugs and cured with prescription drugs. The World Health Organization estimates that world-wide 300-500 million cases of malaria occur and more than 1 million people die of malaria each year. P. falciparum is responsible for many fatalities. About 1,200 cases of malaria are diagnosed in the United States each year. Most cases in the U.S. occur in immigrants and travelers returning from malarial areas, mostly from sub-Saharan Africa and the Indian subcontinent. Malaria is not endemic to Los Angeles County.

Heartworm Heartworm is a filarial worm disease of dogs and cats that is transmitted by the western treehole mosquito. The disease is prolonged and usually fatal due to worm congestion in the heart chambers and vessels. Heartworm symptoms vary with each individual case. In the early stages of infection, dogs and cats appear healthy but can cause damage to internal organs such as the heart, liver, and kidneys. Symptoms are not evident until the advanced stage of the disease. Pets with massive infections can die from failure of the affected organs. The highest incidence of canine heartworm occurs in working and sporting dogs. Dogs and cats that live in areas where heartworm occurs should be regularly checked by a veterinarian. Tests are available to determine if a pet has a heartworm infection. Medications are available from veterinarians to prevent a heartworm infection.

Disease Surveillance

The disease surveillance program serves as an early warning system in the detection of mosquito-borne viruses that can infect people and animals. Testing for the presence of virus/pathogens in mosquitoes, sentinel chickens, and wild birds helps identify the disease transmission cycle before human cases occur.

Mosquito Testing Disease surveillance is conducted to detect St. Louis encephalitis (SLE), Western equine encephalomyelitis (WEE) and West Nile virus (WNV) transmission within District boundaries. Initial surveillance begins by setting mosquito traps to collect adult mosquitoes for identification. Because they require a blood meal to produce eggs and are involved in the disease transmission cycle, only female mosquitoes are submitted for virus testing. Ten to 50 live anesthetized female mosquitoes are deposited into vials to comprise a mosquito pool. These pools are stored at minus 70 degrees Celsius and shipped to the University of Davis Arbovirus Research Unit (DARU) for testing.

New Jersey Light Traps The New Jersey light trap (NJLT) is exclusivly used to estimate mosquito population densities. A NJLT uses an incandescent light source and fan to attract and then suck mosquitoes into a container. NJLT's are not used for disease surveillance. Of these traps, 25 were located throughout the District. In March of 2001, several non-productive trap sites were discontinued because they were not capturing sufficient numbers of mosquitoes due to competing light sources in urban areas. Specimens are collected from the traps weekly during mosquito season and biweekly during the winter.

EVS/CO2 Baited Traps The encephalitis vector surveillance (EVS) traps use dry ice (CO2) as a bait to attract host-seeking female mosquitoes. EVS traps are used mainly to determine whether SLE, WEE or WNV occurs within a localized area. During mosquito season, traps are set overnight once a week between early spring through late summer at several fixed locations throughout the District to capture mosquitoes, particularly Cx. tarsalis and Cx. quinquefasciatus, which are the vectors of SLE, WEE, and WNV .

Reiter Traps The Reiter trap uses a hay infusion mixture to attract gravid (pregnant) blood-fed Cx. quinquefasciatus mosquitoes. Reiter traps are also important in collecting mosquitoes for virus testing, because the blood-fed mosquitoes are more likely to contain virus. Like EVS traps, these traps are placed late in the afternoon and collected the following morning. They are situated in close proximity to the EVS trap sites.

Sentinel Chicken Testing The sentinel chicken flocks also serve as an early warning indicator system for detection of SLE, WEE, and WNV transmitted by mosquitoes that can potentially infect people and animals. Blood samples are taken bi-weekly from the sentinel chickens and analyzed by the state health department's Viral and Rickettsial Disease Lab to determine the presence of virus antibodies, indicating whether mosquitoes in the area have transmitted a virus. This early detection allows the opportunity to prevent the occurrence of human infections. The Vector Ecologists maintain seven chicken flocks throughout the District year-round.

Wild Bird Testing Many wild bird species, such as sparrows and finches, are reservoir hosts for the same mosquito-borne viruses that can infect people. For this reason, they play an important role in the District's disease surveillance program. Blood samples from captured free-ranging wild birds are taken bi-weekly to obtain sera samples and analyzed by the University of Davis Arbovirus Research Unit for the presence of SLE, WEE, and WNV antibodies. The Vector Ecologists also band each captured bird, record biological data, and then release the bird unharmed. Six wild bird traps are maintained year-round in suspect encephalitis transmission foci within District boundaries.

Dead Bird Surveillance Program Wild birds are the animals from which mosquitoes primarily acquire West Nile virus. Although many birds that are infected with WNV will not appear ill, WNV can cause serious illness and death in some birds. The most severe illnesses are seen among the corvids (including crows, jays, ravens, and magpies). Dead American Crows are the most commonly reported bird infected with WNV.

If you find a bird that has recently died, particularly a crow or other corvid, please call the California Department of Health Services hotline at 1-877-WNV-BIRD. Not all birds will qualify for testing; however, they will record all reports of dead birds. To move or discard a carcass, wear rubber or latex gloves. If gloves are not available, use a plastic bag turned inside-out over your hand and invert the bag to surround the bird. Seal and discard in the trash if the bird will not be picked-up for testing.